1. Field of the Invention
The present invention relates generally to sealed electric storage batteries, and more particularly to vent caps for such batteries which provide a flow path for the escape of hydrogen and oxygen formed during the electro-chemical reaction which takes place in such batteries. Still more specifically, the invention relates to a vent cap which also controls the flow of electrolyte which may enter the vent cap to ensure that it is returned to the battery cell and does not flow through the vent cap to the exhaust gas port or become entrained in the flow of gases passing through the vent cap.
2. Description of Related Art
Conventional lead-acid batteries, such as those used in automotive applications, generally include a number of cells disposed in a battery housing. Each cell typically includes a plurality of positive and negative battery plates or electrodes. Separators are sandwiched between the plates to prevent shorting and undesirable electron flow produced during the reaction occurring in the battery. The plates and separators are immersed in a liquid electrolyte in the cell, the most common being aqueous sulfuric acid. The positive plate generally is constructed of a lead-alloy grid covered with lead oxide, while the negative plate generally contains lead as the active material, again covering a lead-alloy grid.
The electromotive potential of each battery cell is determined by the chemical composition of the electro-active substrates employed in the electrochemical reactions. For lead-acid batteries, such as those described above, the potential is usually about two volts per cell, regardless of cell volume. Since vehicles manufactured by original equipment manufacturers (OEMs) typically require 12-volt batteries, most automotive batteries include six cells (6 cells.times.2 volts per cell=12 volts). The size of the housing for the battery is selected based on the packaging constraints of a particular vehicle, i.e., the physical dimensions defined by the vehicle manufacturer for containment of the battery in the engine compartment.
In most battery constructions the battery housing includes a box-like base containing the cell and is made of a moldable resin. The housing is generally rectangular in horizontal cross section, the cells being provided by vertical partitions within the housing. A cover is provided for the casing, the cover includes terminal bushings and a series of fill tubes to allow electrolyte to be added to the cells and to permit servicing, if required, during the life of the battery. To prevent undesirable spillage of electrolyte from the fill tubes, batteries have included some sort of filler hole cap. Battery electrolyte spillage can be caused by a number of factors, including vibration or tilting as the vehicle with which the battery is used maneuvers during normal use. Electrolyte escape may also be caused by battery overheating, a problem especially pronounced in recent years with smaller car engines which tend to create an adverse thermal environment around the battery.
In addition to preventing spillage of electrolyte from the cells, the design of the battery cover and filler caps need to perform an important and different function, namely exhaust of gases generated during the electrochemical reaction. More specifically, gases are liberated from lead-acid batteries during the charge and discharge reactions. Such reactions start at the time the battery is originally charged (called "formation") by the manufacturer or by the retailer or vehicle manufacturer. They also occur during normal operating use of the battery. Factors such as high current charge and discharge conditions, and changes in temperature, can affect the rate at which gas evolution occurs. The gas generation and evolution issues in lead-acid battery construction are particularly important because the liberated gases are hydrogen and oxygen, and it is important to vent such gases in a controlled way from the battery to prevent pressure build-ups in the housing which could lead to electrolyte leaks, housing failures or, most significantly, explosions within the housing.
Electrolyte spillage and gas evolution are interrelated and equally important in the construction of an effective vent cap system. For example, electrolyte may enter the vent cap through several mechanisms. One mechanism is through vibrational or tilting spray of electrolyte into the cap, and another is through a mechanism frequently referred to as "pumping." The latter occurs when gas evolved in the battery bubbles from the cells and carries or forces electrolyte out the fill tube and into the cap. Upon entering the cap, the electrolyte may be carried out the exhaust passage to cause damage to external battery components such as the battery terminals or adjacent engine components.
OEMs have recognized the importance of the dual function performed by the vent caps and have instituted a number of testing specifications designed to ensure electrolyte retention within the cells of the battery. One such test involves tilting a battery thirty-five degrees (35.degree.) about the longitudinal center line of the battery. While a number of different solutions have been proposed to provide an effective vent cap system, optimization has still not been achieved in one vent cap due to numerous demands with which the battery designer is faced--ensuring adequate electrolyte return, condensation, reducing electrolyte in the exhaust flow, pumping of electrolyte through the vent cap system and tilting of the battery. All of these factors can result in electrolyte loss.
An improved vent cap system for minimizing the possibility of electrolyte leakage from the battery and efficiently directing gases from the battery is still needed. Such an improved vent cap would represent a substantial advance in the art.